Rename __dso_handle_so.c to __dso_handle_so.h

Also chang libc/arch-arm/bionic/crtbegin_so.c to include it
as a header.

Change-Id: Ib91b0b8caf5c8b936425aa8a4fc1a229b2b27929

Conflicts:
	libc/arch-arm/bionic/crtbegin_so.c